1. Field of the Invention
The present invention relates to the field of semiconductor memories, particularly to non-volatile memories, and even more particularly, to memories in which operations to modify their content, both by writing and by erasure, are performed electrically. In particular, the present invention relates to EEPROM and flash EEPROM memories.
2. Description of the Related Art
As is known, EEPROM memories are non-volatile semiconductor memories the content of which can be modified electrically, both by writing and by erasure, and in single bytes (eight bits) or, more generally, in single memory words (sixteen bits, thirty-two bits, or more).
Flash memories are also non-volatile semiconductor memories, the contents of which can be modified electrically both by writing and by erasure. However, unlike EEPROM memories, whilst the flash memories, which are currently available, can also be written in single bytes, they can be erased only in blocks the sizes of which are typically from 32 kbytes to 128 kbytes.
The fact that flash memories are widely used is due, to a large extent, to the availability of technological processes, which permit the production of very reliable, large-capacity flash memories, and to the low cost of these memories.
However, for some applications such as, for example, those in the field of smart-card microcontroller systems, the use of flash memories has been excluded up to now. These applications in fact require the provision of non-volatile memories the contents of which can be modified, that is, written and erased, electrically and in single bytes or in single memory words constituted by a few bytes (sixteen or more bits).
For this reason, EEPROM memories are still used in the above-mentioned applications.
However, the use of EEPROM memories cannot be considered satisfactory, particularly in view of future developments of the above-mentioned applications which can be expected to require ever greater memory capacity, together with good reliability and low cost, which is no minor factor. As is known, EEPROM memories are not outstanding with regard to these characteristics.
It would therefore be highly desirable also to be able to use flash memories instead of EEPROM memories in these applications.
Pending European patent application No. 00830553.4, filed by the present Applicant on Feb. 8, 2000 and entitled “A semiconductor memory”, describes a flash memory the contents of which can be altered in single bytes or, more generally, in single memory words, both by writing and by erasure. A memory of this type, which will be referred to below as a “page-flash” memory, could therefore satisfy the requirements of the above-mentioned applications, thus constituting a promising alternative to the use of EEPROM memories.
In any case, both in EEPROM memories and in flash memories, and in particular in the page-flash memory mentioned above, every time it is desired to alter the content by one byte or memory word, that is, in a memory location associated with a certain memory address, the erasure of the pre-existing datum and then the writing of the new datum are performed in sequence.
If, for any reason, for example, owing to a loss of supply voltage or to a malfunction of the memory, the step of writing the new datum cannot be successfully completed, not only is the new datum not stored, but the pre-existing datum is also lost, since it is no longer available.
In many applications, including those already mentioned in the field of smart-card electronic systems, it would, however, be desirable to be able to ensure that, if the step of writing the new datum is not achieved completely and correctly, the pre-existing datum in the memory location addressed is not lost, so that it can be reinstated when the device is restarted.